1. Field of the Invention
This invention concerns inductive components, of the type including one or more windings, and which can be used therefore depending on case as inductors or alternating current transformers. Such components, as inductors, are generally used to perform in electric or electronic circuits the filtering, smoothing or energy storage functions, being conventionally traversed by currents with a DC component on which an AC component is superimposed. A current operating frequency range is 10 kHz to 3 MHz. Such components are for instance currently used in switched power supplies or DC converters. Also, these components are conventionally made so that they can be installed on printed circuits in a manner known itself.
2. Description of the Related Art
Known inductors of the type mentioned above generally consist of one or more enameled copper wire windings made on a toroidal core supported by a base including connecting pins. Conventionally, especially to reduce the overall surface area on the printed circuit, the toroidal windings are arranged vertically on the base so as to extend perpendicularly to the surface of the printed circuit. The ends of the wires are connected to the connecting pins or themselves form the said pins which are intended to be inserted into holes drilled in the printed circuit or soldered to it in a conventional manner. Although it is possible to also adopt a surface-mounted component (SMC) type design which is more suited to automatic installation, the high volume and weight of these components generally prohibits such a design and these components must be mounted manually on the printed circuit before soldering. Also, the mechanical strength in cases of strong vibrations is not very reliable on account of the high weight and the relative distance of the core from the printed circuit when compared with the relatively small dimensions of the base.
Moreover, the magnetic materials used for the toroidal core are generally iron powder based, for example, iron-silicon, when the planned operating frequencies are low, up to 100 kHz, or when the frequencies are higher, up to 200 kHz, made of a ferronickel alloy such as permalloy, for instance the material currently known under the name of Moly-Permalloy or MPP, which is a sintered iron and nickel powder with 80 or 50% nickel.
These two materials have the advantage of supporting a high DC magnetic field which enables the section of the core, and therefore the overall size of the component to be reduced.
However, their losses are high when used at high frequencies, that is around several hundred kHz to several MHz and therefore are poorly suited to uses such as in converter switching power supply circuits which increasingly use very high frequencies.
Another disadvantage of toroidal-type windings is that they are not sealed, the wire being simply wound around the toroidal core without external protection.
The purpose of this invention is to solve these problems and especially aims at supplying an inductive component with a low weight and a low volume, limiting the losses when used at high frequencies and where installation can be facilitated and automated by authorizing the design of these components as surface-mounted components (SMC).
With these targets in mind, the subject of the invention is an inductive component intended to be installed on a printed circuit and including at least one winding consisting of an electrically conductive wire and a magnetic core, characterized in that:
the winding consists of a conductive wire wound in the form of a flat coil the ends of which are connected to the inner ends of the connecting terminals,
a body, formed of a block of insulating material with a lower face more or less orthogonal to the axis of the coil, is overmoulded on the coil and on the said inner ends of the terminals, the body including a central opening which passes through it along the axis of the coil,
the core is made of ferrite and surrounds the body in the centre plane containing the axis of the coil and has a centre element passing through the opening of the body.
The combination of characteristics according to the invention especially has the advantage of providing a significant gain in volume and in weight when compared with inductive components with equivalent properties made in the form of toroidal core inductors: a component according to the invention takes up, for instance, a volume of 1200 mm3 whereas an equivalent inductor with a toroidal core takes up a volume of around 3240 mm3. These advantages result especially from the use of a winding with a low height and of a ferrite magnetic core which, thanks to its magnetic characteristics, enables a reduction in the section. Ferrites have low losses at high frequencies and such a material is therefore especially suitable for the applications targeted by the component according to the invention, that is for frequencies of up to 3 MHz, such as, for example, converter switching power supplies where the switched frequencies tend to be increasingly higher. Also, the low height of the component enables a reduction in the overall thickness of the printed circuit on which it is mounted.
The body, for example made of a thermosetting epoxy resin, overmoulded directly on the coil and the connections, provides high mechanical strength, good dissipation of the losses generated by passing the current through the winding and good sealing enabling the component to be used in wet environments. The fact of not including the ferrite core in the moulding but adding it around the body, and externally apparent, improves still further the dissipation of the thermal energy generated especially by the eddy currents this thanks to direct contact of a large external surface area of the core with the exterior and the possibility of easily associating a heat sink.
According to a specific arrangement of the invention, the core consists of two elements extending respectively on each of the faces of the body, one at least of the said elements being E-shaped the centre arm of which passes through the opening of the body and the outer arms of which pass on two opposite sides of the said body. This arrangement offers, at same volume and when compared with the use of ferrite cores made in known forms, for instance a toroidal form, a much higher iron section. For an equivalent induction level, the number of turns of the winding can therefore be reduced which reduces the losses in the conducting wire and therefore enables a higher current.
This design of the ferrite cores also enables an air gap to be easily made in the magnetic circuit between the two elements comprising the core, at the level of the outer faces of at least one of the arms of the E. This air gap can be adapted for instance by playing on the respective lengths of the arms of the E. This air gap enables the core to support a high DC field and, correlatively, for a given field, a reduction in the volume of the core.
Preferably, the two elements of the core are bonded to each other when they are installed on either side of the body. The adhesive joint, made by a non-magnetic adhesive at the interface between the two elements of the core can moreover be placed in the air gap mentioned above at the level of one or more of the arms of the E. The securing of the core on the body can be completed by an additional adhesive joint placed between the edges of the elements of the core and the body, in particular, on the sides of the component.
According to another specific arrangement, the connecting terminals emerge from the body at the level of the lower face of the body, on two opposite sides of the body in relation to the said centre plane. These terminals are secured to the body by overmoulding. The outer ends of these terminals may be shaped to form pins for conventional installation on printed circuits. They will however preferably be shaped so as to form lugs extending in the plane of the lower surface of the body or slightly prominent, enabling the component to be attached to the printed circuit by the soldering of these lugs to the surface of the said circuit according to the technique known for SMCs.
The low height of the component associated with much larger transverse dimensions, especially the distance between the lugs located on each side of the component, and the low weight considerably improve the vibration resistance when the component is soldered to the circuit.
The lugs, in addition to ensuring a mechanical attachment function to the printed circuit by soldering, at least those to which the ends of the winding or windings are connected are used of course for their electrical connections. Note, on this subject, a specific advantage resulting from the SMC-type design according to the invention which lies in the large contact surface area possible between the lugs and the printed circuit which enables very low connection resistances and high currents to be obtained. This advantage is even more marked when, as can be achieved when the component includes only a single winding, this winding is connected to connections which extend along the complete length of the sides of the component.
Again, another advantage of the inductive components according to the invention is that they can be packed in strips for use by automatic installation machines, their flattened format and their low weight authorizing automatic installation by suction or by grips.
The subject of the invention is also a manufacturing process for an inductive component intended to be installed on a printed circuit and including at least one winding and a magnetic core, this process being characterized in that:
the winding is made in the form of a flat coil by winding a wire without using a former,
the winding is placed on a grid, the axis of the winding being perpendicular to the grid, and the ends of the wire are soldered to the said grid,
a body made of an insulating material is overmoulded onto the assembly thus obtained to leave a central opening at the axis of the coil and leaving the edges of the grid apparent on two opposite sides of the body,
two ferrite core elements are placed on either side of the body at least one of which is E-shaped, the centre arm of the E being inserted in the said central opening of the body and the two other arms passing on two opposite sides of the body and the two elements of the core are fixed to each other.
Preferably, the winding is made with a wire including an outer thermobonding layer and, after winding, an electric current of sufficient amperage is passed through the wire to heat it and to obtain the bonding of the turns to each other.